THE HIPPOPOTAMUS. 145 



sided shield, and has a wart-shaped protuberance 

 between the two halves both out and inside. Each 

 half consists of two almost three-sided tubercles, 

 which lie flat against the inner sides that are 

 turned towards one another. From this it is 

 evident that the enamel outline of the used tooth 



FIG. 18. Second Lower Molar of the Eiver-horse, to the right. 

 Nat. size. 



(Fig. 19) consists originally of two pairs, with the 

 basal line of the three corners turned towards one 

 another, and which do not coalesce till they have 

 been used for a long time. 



The Hippopotamus has become almost an 

 amphibian in its mode of life; its skull and 



